1. Field of the Present Invention
The present invention relates generally to fluid level sensors often used to detect the level of liquids in vehicle sumps and, more particularly, to means for mounting piezoelectric transducers in a sensor housing.
2. Description Of The Prior Art
There are many known types of liquid level sensors. However, it has been determined that ultrasonic level sensors provide several advantages over other types of sensors such as float actuated sensors. The application of an electrical voltage across the plates of a piezoelectric member induces a mechanical vibration. The vibration is transferred through a flow path between a transmitting transducer, and a receiving transducer, whereby the receiver is vibrated to induce a responsive voltage across the plate of the transducer.
The transducers are vibrated at or within a range near a resonant frequency so that the amplitudes of the vibration and the resulting electrical signal are maximized. Furthermore, the greater the physical connection through the connecting path, the greater the response signal which is generated. Of course, if the oscillation is affected by damping from mounting or frequent vibration, both the vibration amplitude and electrical signal strength can be substantially reduced. Moreover, mass production techniques often provide inconsistencies in the circuit construction, and the fluid sumps of motor vehicles often provide an unstable environment for the sensors. For example, performance can vary widely due to changes in the environment surrounding the sensor including movement of the fluid and temperature conditions, as well as quality control of the transmission or reception circuitry coupled with the sensor. Such factors combine with the previously known mounting arrangements of piezoelectric members in the sensor housing to provide varying performance characteristics when the units are installed in various installations.
For example, U.S. Pat. No. 4,063,457 to Zekulin et al discloses an ultrasonic sensing device in which each pair of a plurality of transducer assemblies is mounted to a set of parallel walls of a metallic cylinder. The sensors may be secured directly against the metallic housing. Alternatively, the crystal is seated on a number of rubber feet and then adhered in position with a thin layer of a conventional epoxy resin. However, each of these methods of construction may interfere with the signal which can be transmitted or dampen the transmission or reception of vibration. Moreover, while such damping may be relatively small, it may be compounded by loss of signal strength due to frequency changes or environmental conditions.
Moreover, a previously known construction for mounting piezoelectric members in a sensor housing in a manner which purports to avoid such problems is subject to laborious or complicated construction techniques. For example, U.S. Pat. No. 4,703,652 to Itoh et al requires a substantial number of connections between conductors, fine wires, base members, caps, piezoelectric members and the like. The assembly of such a large number of parts within a small space can be quite difficult and is especially labor intensive and time consuming. In particular, brazing and soldering of the wire connections as taught in the patent require particular handling and care.